1. Technical Field
The present invention generally pertains to the monitoring of ground water quality. More specifically, the present invention relates to a method for monitoring ground water purged using low-flow techniques wherein the stabilization of select water parameters is monitored and analyzed.
2. Discussion
Heightened awareness of the dangers of subsurface water pollution has resulted in the development of improved techniques for obtaining and analyzing subsurface water quality. Originally, water to be analyzed was purged from a well casing using bailers or high speed pumps. Using this method three to five casing volumes of water were removed from a well, and from this water the sample to be analyzed was collected. However, this method often adversely impacted upon sample quality due to the collection of samples with high levels of turbidity. Such samples often included immobile foreign particles which were not representative of the water in the vicinity of the well and which produced an overestimation of certain substances of interest, such as metals or hydrophobic organic compounds.
In an attempt to rectify the turbidity problem, filtration was often implemented with less than desirable results. Filtration was found to remove potentially mobile contaminant-associated particles, thus resulting in artificially low contaminant concentrations. It is now generally accepted that water in a well casing is not necessarily representative of the water generally contained in the ground surrounding the well, which is known as the formation water. However, it has been found that water within the screened interval of a well may be representative of the formation water.
The emergence of low-flow purging and sampling techniques have substantially diminished the turbidity problems described above and minimized the mixing between the overlying stagnant casing water and the water within the screened interval. Low flow does not refer to the flow rate of water discharged at the surface, but instead refers to the velocity with which water enters the pump intake. Low flow purging is typically performed using a pump-intake located in the middle or slightly above the middle of the screened interval of the well. This is done because if the pump is placed too close to the bottom of the well, the purging will cause the unwanted uptake of solids which have collected at the bottom of the well.
In order to ensure that the water pumped from the well is representative of the formation water rather than the stagnant water of the well casing, various parameters of the purged water may be monitored. Parameters that are often monitored are pH, temperature, specific conductance, oxidation-reduction potential, dissolved oxygen and turbidity. Stabilization of these factors indicates that the purged water is representative of the formation water. Measurements are usually taken every three to five minutes and stabilization is achieved after all parameters have stabilized for a sufficient duration. The parameters will be considered stabilized if preferably about three successive readings are within plus or minus about 0.1 for pH, about 3% for conductivity, about 10 mV for redox potential, and about 10% for turbidity and dissolved oxygen.
The monitoring of the above parameters is a very useful tool in determining when the pump has begun to discharge samples representative of the formation water. However, determining when stabilization has been achieved is difficult, requiring continuous monitoring of multiple parameters and repeated calculations. Consequently, there exists the need for a device which can automatically monitor the parameters and provide notification when the selected parameters have stabilized.
It is a general object of the present invention to provide a method of monitoring purge water quality. It is another object of the present invention to provide a method for monitoring select purge water parameters.
In one preferred form, the present invention provides a method for monitoring select purge water parameters and determining when the parameters have stabilized. The method includes the steps of selecting which parameters are to be monitored and selecting the time interval between which evaluation readings are performed. The time interval may be selected in set time increments from within a predetermined time range. With the passing of each interval, the results of the evaluation readings are stored in a memory device for at least one, and more preferably a plurality, of consecutive readings. If the readings do not fall within the defined stabilization ranges for the selected parameters, the oldest scan is deleted from the memory device. The comparison of the reading results is again performed with the start of the next interval, with the new reading results being compared to the two old reading results. When the selected parameter values do fall within the defined ranges for three consecutive readings, the operator is alerted that stabilization has been obtained.